Polylactic acid is a plant-derived biomass polymer, and is drawing attention as a resin replacing the petroleum-derived polymers.
However, although polylactic acid (in the present invention, to be said poly-L-lactic acid having L-form of optical isomer as main component) is a crystalline polymer, its crystallization speed is extremely slow, and polylactic acid is hardly crystallized by a method comprising nipping with cooling rolls after melt film formation which is a general film formation condition.
Therefore, improvement of heat resistance of polylactic acid based films has been conventionally tried by some methods as mentioned below.
For example, after sheeting according to a melt extrusion method and the like, stretch-oriented crystallization is performed by biaxial stretching to express heat resistance during film formation of polylactic acid (patent document 1).
However, due to a residual internal stress from stretching in this method, heat shrink defectively becomes extremely high when high temperature is used. Therefore, the temperature that can be actually used is about 100° C. at most.
In addition, an attempt has been made to express heat resistance by blending other high melting point materials with polylactic acid (patent document 2).
In this method, however, problems of decreased ratio of plant-derived components (biomass ratio), decreased transparency and the like occur.
In the field of molding materials, an attempt has been actively made to form in a short time at a low metal mold temperature by increasing the crystal growth rate by the addition of a crystal nucleating agent and the like. In the case of film formation, however, the film is generally cooled to not more than the glass transition temperature immediately after melt film formation, so as to maintain the shape of the film. Since a film is thin as compared to molded parts and the cooling rate becomes high by this cooling method. Thus, addition of a useful nucleating agent is hardly effective.
As for this problems, promotion of crystallization by providing a heating step at 60 to 100° C. in a step after film formation has been suggested (patent document 3). This temperature range is also described to aim at suppression of deformation of a formed film due to detachment failure from a metal roll of polylactic acid.
However, this method is inefficient since heating is applied again after cooling once for solidification.